1. Technical Field
The present disclosure generally relates to the field of illumination devices and, more particularly, to control dusk-to-dawn illumination.
2. Description of the Related Art
Energy conservation has become of ever increasing importance. Efficient use of energy can result in a variety of benefits, including financial benefits such as cost savings and environmental benefits such as preservation of natural resources and reduction in “green house” (e.g., CO2) gas emissions.
Residential, commercial, and street lighting which illuminate interior and exterior spaces consume a significant amount of energy. Conventional lighting devices or luminaires exist in a broad range of designs, suitable for various uses. Lighting devices employ a variety of conventional light sources, for example incandescent lamps, florescent lamps such as high-intensity discharge (HID) lamps (e.g., mercury vapor lamps, high-pressure sodium lamps, metal halide lamps).
There appear to be two primary approaches to reducing energy consumption associated with lighting systems. One approach employs higher efficiency light sources. The other approach selectively provides light only when needed.
Use of higher efficiency light sources may, for instance, include replacing incandescent lamps with florescent lamps or even with solid-state light sources (e.g., light emitting diodes (LEDs), organic LEDs (OLEDs), polymer LEDs (PLEDs)) to increase energy efficiency. In some instances, these higher efficiency light sources may present a number of problems. For example, florescent light sources take a relatively long time after being turned ON to achieve their full rated level of output light or illumination. Such light sources also typically have a high energy consumption during warm-up. Many higher efficiency light sources emit light with a low color rendering index (CRI). For reference, sunlight has a CRI of 100 and represents “ideal light” which contains a continuous spectrum of visible radiation. Low CRI light is less pleasing to the human eye. Surfaces illuminate with low CRI light may not be perceived in their “true” color. Low CRI light makes it more difficult to discern details, often requiring a higher level of output light or illumination to discern details that would otherwise be discernable in high CRI light. Further, higher efficiency light sources may require additional circuitry (e.g., ballasts) and/or thermal management techniques (e.g., passive or active cooling).
Providing illumination only when needed can be achieved manually by a user of the lighting system, or automatically through the use of one or more control mechanisms. Automatic control mechanisms generally fall into two broad categories, timers and environmental sensors. Timer based control mechanisms turn light sources ON and OFF based on time. The times are typically user configurable and result in the luminaire turning ON for a period of time and then OFF for the remainder of a 24 hour period. Such timing circuits rely on the user to account for changes in length of daylight which may occur throughout a year by adjusting the ON period of the luminaire commensurate with the change in day length. Very often, timer based control mechanisms are set once and never updated.
Automatic control devices such as photosensitive transducers and motion or proximity sensors add to the cost of a light fixture and are frequently mounted in exposed positions where environmental or physical damage is unavoidable or vandalism may occur. In addition, a failure of the automatic control mechanism, for example failure of a photosensor used to turn the light source ON or OFF dependent upon the measured ambient light level may result in the light source remaining in a continuously ON state in the event the automatic control mechanism fails in a “closed” position permitting current flow to the light source or in a continuously OFF state in the event the automatic control mechanism fails in an “open” position interrupting current flow to the light source. Either failure mode results in an unacceptable mode of operation of the light source.
New approaches to improving the automated performance of lighting systems in the absence of one or more photosensitive devices are therefore needed.